1. Field of the Invention
The present invention relates to an electret-type capacitor microphone, and more particularly to a structure of a thin electret-type capacitor microphone.
2. Description of the Related Art
Conventionally, an electret-type capacitor microphone is known as a type of microphone.
As disclosed in, for example, Japanese Utility Model Registration No. 2587682, this electret-type capacitor microphone is arranged such that a capacitor portion and a substrate are accommodated in a hollow cylindrical case whose one end has an end face wall defining an opening for a sound hole and whose other end is open. The capacitor portion is formed by a diaphragm and a back plate opposing to each other with a spacer placed therebetween and a substrate has an impedance conversion element (active element) mounted thereon for converting a change of the electrostatic capacity of the capacitor portion into electric impedance.
As disclosed in the aforementioned publication as well, the arrangement often adopted is such that the capacitor portion is accommodated with the diaphragm positioned on the one end side of the case, while the substrate is accommodated on the other end side of the case relative to the capacitor portion with the impedance conversion element facing the capacitor portion.
The electret-type capacitor microphone can be relatively easily constructed with a compact size. However, when the electret-type capacitor microphone is installed in a cellular phone or the like which tends to be increasingly thinner in recent years, it is desirable to not only make the capacitor microphone compact but also make as thin as possible.
However, with the above-described conventional electret-type capacitor microphone, an interval somewhat larger than the height of the impedance conversion element needs to be secured between the back plate and the substrate in order to prevent interference between the back plate of the capacitor portion and the impedance conversion element. For this reason, there is a problem in that the capacitor microphone cannot be made very thin, therefore making it impossible to cope with the demand for the thinner capacitor microphone in recent years.
The invention has been made in view of the above-described circumstances. An object of the present invention is to provide an electret-type capacitor microphone which can be made sufficiently thin.
In the present invention, the above object is attained by forming a predetermined communicating opening in the back plate.
According to the present invention, there is provided a capacitor microphone comprising:
a case having an end wall defining an opening for a sound hole;
a capacitor portion comprising a diaphragm, a back plate and a spacer, the diaphragm opposing to the back plate via the spacer, the capacitor portion being accommodated in the case in such an manner as the diaphragm is placed on the side of the end wall; and
a substrate accommodated in the case farther from the end wall than the capacitor portion, the substrate having an impedance conversion element mounted on a surface thereof facing the capacitor portion,
wherein a communicating opening is provided around the back plate in a position opposing to the impedance conversion element for communicating spaces on the front and back sides of the back plate; and
the communicating opening has a larger area than that of the top of the impedance conversion element.
In the capacitor microphone, the top of the impedance conversion element is inserted into the communicating opening.
In the capacitor microphone, the back plate has a recessed outer peripheral surface, and the communicating opening is provided between the recessed outer peripheral surface of the back plate and the inner peripheral surface of the case.
In the capacitor microphone, at least one terminal pin is mounted on the substrate so that the head of the pin faces the capacitor portion,
another communicating opening is provided around the back plate in a position opposing to the head of the pin for communicating spaces on the front and back sides of the back plate, and
the communicating opening has a larger area than the head of the pin.
The above-described xe2x80x9ccapacitor microphonexe2x80x9d may be a foil electret-type capacitor microphone in which the diaphragm functions as an electret, or may be a back electret-type capacitor microphone in which the back plate functions as an electret.
As for the xe2x80x9copening for a sound hole,xe2x80x9d insofar as it is an opening which functions as a tone hole, the opening itself may constitute the sound hole, or the sound hole may be formed by inserting and fitting a separate member in the opening.
As for the xe2x80x9cimpedance conversion element,xe2x80x9d insofar as it is capable of converting a change of the electrostatic capacity of the capacitor portion into electric impedance, the impedance conversion element is not limited to a particular element. For example, a junction field-effect transistor (JFET) or the like may be adopted.
As for the xe2x80x9ccommunicating opening,xe2x80x9d insofar as it has an opening area larger than the area of the upper end portion of the impedance conversion element, its size, shape and the like are not particularly limited. The xe2x80x9ccommunicating openingxe2x80x9d may be provided in the back plate alone or may be formed in the back plate and another member.
According to the capacitor microphone according to the invention, the communicating opening having the opening area larger than the area of the upper end portion of the impedance conversion element to allow spaces on both sides of the back plate to communicate with each other is formed in the back plate at the position opposing the impedance conversion element. Accordingly, it is possible to ensure that the impedance conversion element and the back plate do not interfere with each other, even if an interval somewhat larger than the height of the impedance conversion element is not secured between the back plate and the substrate in the conventional manner.
Therefore, according to the invention, it is possible to make a thin capacitor microphone sufficiently.
It should be noted that, as also disclosed in the aforementioned publication, in a case where the diaphragm of the capacitor portion is positioned on the one end side of the case relative to the back plate, a measure is generally taken to expand the acoustic space inside the case by forming in the back plate a plurality of small holes for allowing the spaces on both sides of the back plate to communicate with each other. In the present invention, the aforementioned communicating opening can be made to function as the plurality of small holes.
In the above-described construction, if an arrangement is provided such that the upper end portion of the impedance conversion element is inserted in the communicating opening, the capacitor microphone can be made even thinner. This being the case, however, even if the distal end portion of the impedance conversion element is not inserted in the communicating opening, since the opening area of the communicating opening is larger than the area of the distal end portion of the impedance conversion element, the interval between the back plate and the substrate can be made narrower than in the conventional arrangement without causing interference between the impedance conversion element and the back plate.
As mentioned above, the specific arrangement of the aforementioned xe2x80x9ccommunicating openingxe2x80x9d is not particularly restricted, but if an arrangement is provided such that the communicating opening is formed between an inner peripheral surface of the case and an outer peripheral surface of the back plate formed in such a manner as to be indented toward an inner peripheral side thereof in relation to the inner peripheral surface, the communicating opening can be disposed at a position sufficiently spaced apart from the center of the diaphragm. Consequently, the capacitor microphone can be made thin without adversely affecting the acoustoelectric conversion characteristic of the capacitor microphone.
It should be noted that as the arrangement of a plurality of terminals provided in the capacitor microphone, some or all of the plurality of terminals may be constituted by terminal pins, and heads of the terminal pins may project toward the other end side of the case, and maybe fixed to the substrate. However, if this arrangement is adopted, there is a possibility of the terminal pins interfering with the back plate depending on the sizes of the heads of the terminal pins. Accordingly, if a communicating opening having an opening area larger than the area of the head of the terminal pin to allow the spaces on both sides of the back plate to communicate with each other is formed in the back plate at a position opposing the head of the terminal pin, it is possible to prevent the interference with the back plate.
This communicating opening for preventing the interference with the terminal pin maybe the same communicating opening as the communicating opening for preventing the interference with the impedance conversion element, or may be formed separately from it. Furthermore, in the case where there are a plurality of terminal pins, the communicating openings may be formed for the respective terminal pins, or a common communicating opening may be formed for some or all of the terminal pins.
It should be noted that, in this case as well, if the arrangement provided is such that the distal end portions of the heads of the terminal pins are inserted in the communicating openings, the capacitor microphone can be made thin even more easily.